Lamps capable of producing multiple colors of light are known to satisfy many applications; including lamps for general purpose lighting that allow “white” light to be generated in such a way to allow the user to adjust the correlated color temperature of the light. Lights with adjustable color temperature are further known for use in specialized lighting applications, such as camera strobes and motion picture lighting systems. Within this application space, it is most desirable to create lamps that provide outputs having both colorimetric coordinates and spectral power distributions that match those of typical blackbody radiators, typical daylight lighting, or standard daylight sources. The colorimetric coordinates of natural light that exists during the day typically fall near a curve, referred to as the Planckian Locus or black body curve, within CIE chromaticity space. Methods for calculating daylight spectra for color temperatures between 4000K and 25000K have been specified within the art (Commission Internationale de l'Eclairage publication No. 15, Colorimetry (Official Recommendations of the International Commission on Illumination), Vienna, Austria, 2004.). Standardized lighting conditions that are desirable to attain and fall near this curve; include those designated D50, D65, and D93, which correspond to daylight color temperatures of 5000K, 6500K, and 9300K, as well as so-called warmer lights, having lower correlated color temperatures, which are more similar in appearance to the light produced by tungsten lamps. In addition to having a lamp that is able to create light having the same colorimetric coordinates as these standardized lighting conditions, it is desirable to have a lamp that produced light having a spectral power distribution that matches the standardized spectral power distributions of these standardized light sources. One metric of the degree of match between the spectral power distribution of the light produced by a lamp and the spectral power distribution of these standard lighting conditions is a metric referred to as the CIE color rendering index or CRI (Commission Internationale de l'Eclairage publication No. 13.3, Method of Measuring and Specifying Color-Rendering of Light Sources, Vienna, Austria, 1995.).
CRI provides a standard method of specifying the degree to which the color appearance of a set of standard reflective objects illuminated by a given lamp matches the appearance of those same objects illuminated by light having the spectral power distribution to a specified standard source. Generally, lamps having a CRI of 80 or better provide a good match to the target spectral power distribution and are deemed to be of high quality.
Lamps known in the prior art that are capable of color control are constructed from at least three different, independently controlled light sources. Noh, in EP Patent 1 078 556, discuss a lamp created using three different fluorescent tubes, a rectifier, three ballasts and a controller for controlling the ballasts. Within this system, the illumination level of the three lamps is controlled to affect a change in the color temperature of the general purpose lighting device. However, this embodiment requires three different light sources that must all perform to a specified level and generate a specified spectral power distribution and be controlled independently. This control mechanism can be complex and is prone to error as each of the lamps age.
This disclosure also discusses the fact that the controller should be configured such that it has two perpendicular axes of control such that one axis ideally represents the luminance level of the lamp and the second axis represents the color temperature of the lamp. However, the authors fail to address the issue that by having three independent light sources, the chromaticity coordinates of the light that will be generated is likely to fall within a two-dimensional triangle in CIE chromaticity space with each corner of the triangle being represented by the chromaticity coordinate of each of the light sources. As such, the system will produce light with chromaticity coordinates which fall on the Planckian Locus only when the CIE chromaticity coordinates of the three light sources is positioned to span the Planckian Locus and the relative luminance produced by each of the three light sources is such that the mixture results in a light with chromaticity coordinates that fall on the Planckian Locus. For this reason, if any one of these lamps ages at a different rate than another (i.e., its spectral power distribution changes, or its luminance output decreases at a different rate than the others), the light output of the lamp is not only unlikely to have a desired color temperature but the chromaticity coordinate of the light output is unlikely to fall near the Planckian Locus and therefore the luminance and the position of the chromaticity coordinate of the lamp's output is unlikely to fall upon the Planckian Locus. Further, to control the illumination from the three sources to create light with chromaticity coordinates that fall near the Planckian Locus as the two controls are manipulated it is necessary to employ a microprocessor or similar device capable of determining the correct mixture of the three sources to create the correct color of light output. The need for a microprocessor can add significant cost and complexity to the overall system design.
Lamps having color temperature regulation have also been discussed by Okumura in US Publication 2004/0264193, entitled “Color Temperature-Regulable LED Light”. Within this disclosure, at least two different embodiments of different, independently-addressable, LEDs are employed. In a first embodiment, a white LED, which is typically formed from a phosphorescent substance that emits broadband light when excited by a blue or ultraviolet emitter is employed together with typical narrow-band blue and yellow LEDs. Within this embodiment, the light produced by the phosphorescent material is a spectrally broadband emission necessary to have a reasonable color rendering index with respect to daylight. The light from the blue and yellow LEDs is then mixed with the broadband light. Different ratios of the blue and yellow LED light in the mixture are used to adjust the color temperature of the overall combination. In a second embodiment, three LEDs are employed, again with at least one of these having a phosphor coating to produce broadband light, one having a blue emission, a second having a yellow emission and a third having an orange emission. Each of these embodiments again describes a lamp employing at least three emissive LEDs, the light from which must be mixed to produce the intended light output. It is important to note that the first embodiment provided within this disclosure employs three LEDs, the colorimetric coordinates of which all are discussed as lying near a single line through CIE chromaticity space. This fact reduces the tendency of the color of the light to shift away from the Planckian Locus if one LED fades faster than another since they lie along a line that is nearly parallel to the Planckian Locus. Unfortunately, this embodiment requires that either the blue or yellow LED be employed with the white LED. Therefore, if the system is not calibrated properly or if one of the LEDs ages at a different rate than another, it is likely that a luminance shift will occur at the point where one LED is turned off and the other is turned on. This has the potential to create a discontinuous change in color temperature as well as a sudden perceptual change in the perceived brightness of the lamp.
Duggal in U.S. Pat. No. 6,841,949, entitled “Color tunable organic electroluminescent light source” also provides for a color tunable lamp. This lamp, however, employs three, independently-addressable, colored light-emitting elements to provide the necessary color range. Once again, the presence of three (e.g., red, green and blue), independently-addressable light-emitting elements within the lamp to allow the lamp to produce light having a range of CIE chromaticity coordinates, requires complex control of the proportion of light from the three, independently-addressable, light-emitting elements to create the exact color coordinates of daylight sources, and factors such as unequal aging of the three lamps makes formation of daylight colors even more difficult. Further, Duggal does not discuss a means for replicating the spectral power distribution of daylight sources through application of the red, green, and blue light-emitting elements.
The use of OLEDs capable of generating different colors of light is well known and devices having three or more colors of light-emitting elements that are either arranged spatially on a single plane as discussed by U.S. Pat. No. 5,294,869 issued to Tang and Littman, entitled “Organic electroluminescent multicolor image display device,” or are composed of a number of stacked, individually-addressable emissive layers as has been discussed by U.S. Pat. No. 5,703,436 issued to Forrest et al., entitled “Transparent Contacts for Organic Devices” have been discussed extensively in the literature. It is further known to create an OLED device having four colors of light-emitting elements by employing light-emitting diode devices with color filters filtering at least some of the light-emitting diode devices, as described in US Patent Application US2004/0113875, assigned to Miller et al., and entitled, “Color OLED display with improved power efficiency”. However, a simple OLED-based lamp has not been provided that allows the continuous change of light color along a single line within the CIE Chromaticity space and that continually provides light output along this single line, even as the lamp ages and the light output of one of the light-emitting elements degrades at a different rate than another.
Other LED technologies are also known for producing light to be used in general purpose lighting environments. For example, studies have been published that demonstrate the ability to stack multiple layers of quantum dots, the individual layers being tuned to complementary wavelength bands, and thereby achieve the emission of white light. For example, in the article “From visible to white light emission by GaN quantum dots on Si (111) substrate” by B. Damilano et al. (Applied Physics Letter vol. 75, p. 962, 1999), the use of GaN quantum dots on an Si(111) substrate to effect continuous tuning from blue to orange by control of the QD size is demonstrated. A sample containing four stacked planes of differently sized QDs was shown to produce white light, as demonstrated via photoluminescence spectral power distribution. Electroluminescent white light emission was not demonstrated, nor was continuous color tuning with a fixed material set.
US 2006/0043361 by Lee et al., discloses a white light-emitting organic-inorganic hybrid electroluminescence device. The device comprises a hole-injecting electrode, a hole-transport layer, a semiconductor nanocrystal layer, an electron transport layer and an electron-injecting electrode, wherein the semiconductor nanocrystal layer is composed of at least one kind of semiconductor nanocrystals, and at least one of the aforementioned layers emits light to achieve white light emission. The semiconductor nanocrystal layer of this device may also be composed of at least two kinds of nanocrystals having at least one difference in size, composition, structure or shape. Organic materials are employed for the transport layers, whereas inorganic materials are employed for the nanocrystals and the electrodes. While such a device may be used to create white light, it does not address the need to vary the color of this white light source or to control the spectral power distribution of the white light source.
U.S. Pat. No. 7,122,842, by Hill, discloses a light emitting device that produces white light, wherein a series of rare-earth doped group IV semiconductor nanocrystals are either combined in a single layer or are stacked in individual RGB layers to produce white light. In one example, at least one layer of Group II or Group VI nanocrystals receives light emitted by the Group IV rare-earth doped nanocrystals acting as a pump source, the Group II or Group VI nanocrystals then fluorescing at a variety of wavelengths. This disclosure also does not demonstrate color tuning during device operation.
US 2005/0194608, by Chen, discloses a device having a broad spectral power distribution Al(1-x-y)InyGaxN white light emitting device which includes at least one blue-complementary light quantum dot emitting layer having a broad spectral power distribution and at least one blue light emitting layer. The blue-complementary quantum dot layer includes plural quantum dots, the dimensions and indium content of which are manipulated to result in an uneven distribution so as to increase the FWHM of the emission of the layer. The blue light-emitting layer is disposed between two conductive cladding layers to form a packaged LED. Various examples are described in which the blue-complementary emission is achieved by means of up to nine emitting layers to provide a broad spectral distribution, and the blue emission is achieved by up to four blue emitting layers. The author also discusses the ability to tune the spectral power distribution and the color temperature of the LED through changing the materials from which the LED is constructed. However, the author does not provide a method for dynamically adjusting the color temperature or spectral power distribution of the device.
There is a need, therefore, for a lamp that provides multiple colors of light output and specifically multiple colors of light output with chromaticity coordinates that lie on or near the Planckian Locus, that is less complex to manufacture, and simpler than the prior art solutions to control while still having the capacity to create light output with an acceptable CRI as compared to standard illumination sources.